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We investigated structures, vibrational frequencies, and rotational Ž . Ž . barriers of disilane Si H , hexafluorodisilane Si F , and hexamethyldisilane 2 6 2 6 Ž . Si Me by using ab initio molecular orbital and density functional theories. We 2 6 Ž employed four different levels of theories i.e., HFr6]31G*, MP2r6]31G*, . BLYPr6]31G*, and B3LYPr6]31G* to optimize the structures and to calculate Ž . the vibrational frequencies except for Si Me at MP2r6]31G* . MP2r6]31G* 2 6 calculations reproduce experimental bond lengths well, while BLYPr6]31G* calculations largely overestimate some bond lengths. Vibrational frequencies Ž . from density functional theories BLYPr6]31G* and B3LYPr6]31G* were in reasonably good agreement with experimental values without employing ‡Ž . additional correction factors. We calculated the DG 298 K values of the internal rotation by correcting zero-point vibration energies, thermal vibration energies, and entropies. We performed CISDr6]31G*rrMP2r6]31G* ‡Ž . calculations and found the DG 298 K values for the internal rotation of Si H , 2 6 Si F , and Si Me to be 1.36, 2.06, and 2.69 kcalrmol, respectively. The 2 6 2 6 Ž . performance of this level was verified by using G2 and G2 MP2 methods in ‡Ž . Si H . According to our theoretical results, the DG 298 K values were 2 6 ‡Ž . marginally greater than the DE 0 K values in Si F and Si Me due to the 2 6 2 6 ‡Ž . ‡Ž . contribution of the entropy. In Si H the DE 0 K and DG 298 K values were 2 6 coincidently similar due to a cancellation of two opposing contributions between zero-point and thermal vibrational energies, and entropies. Our calculated Correspondence to: S. G. Cho ( ) Journal of Computational Chemistry, Vol. 18, No. 12, 1523 ]1533 1997 Q 1997 John Wiley & Sons, Inc. CCC 0192-8651 / 97 / 121523-11

Rotational barriers of disilane, hexafluorodisilane, and hexamethyldisilane: Ab initio, density functional, and molecular mechanics (MM3) studies